Reuniting the Ötztal Nappe: the tectonic evolution of the Schneeberg Complex

The Ötztal Nappe in the Eastern Alps is a thrust sheet of Variscan metamorphic basement rocks and their Mesozoic sediment cover. It has been argued that the main part of the Ötztal Nappe and its southeastern part, the Texel Complex, belong to two different Austroalpine nappe systems and are separated by a major tectonic contact. Different locations have been proposed for this boundary. We use microprobe mapping of garnet and structural field geology to test the hypothesis of such a tectonic separation. The Pre-Mesozoic rocks in the area include several lithotectonic units: Ötztal Complex s.str., Texel Complex, Laas Complex, Schneeberg Complex, and Schneeberg Frame Zone. With the exception of the Schneeberg Complex which contains only single-phased (Eoalpine, i.e. Late Cretaceous) garnet, all these units have two-phased garnet with Variscan cores and Eoalpine rims. The Schneeberg Complex represents Paleozoic sediments with only low-grade (sub-garnet-grade) Variscan metamorphism which was thrust over the other units and their Mesozoic cover (Brenner Mesozoic) during an early stage of the Eoalpine orogeny, before the peak of Eoalpine metamorphism and garnet growth. Folding of the thrust later modified the structural setting so that the Schneeberg Thrust was locally inverted and the Schneeberg Complex came to lie under the Ötztal Complex s.str. The hypothesized Ötztal/Texel boundaries of earlier authors either cut across undisturbed lithological layering or are unsupported by any structural evidence. Our results support the existence of one coherent Ötztal Nappe, including the Texel Complex, and showing a southeastward increase of Eoalpine metamorphism which resulted from southeastward subduction.Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659Rheinische Friedrich-Wilhelms-Universität Bonn (1040